During the last decade, somatosensory evoked potentials (SSEP) have become established as a practical method for monitoring the spinal cord during various surgical procedures where there is a risk of paraplegia, e.g., scoliosis surgery, thoracic aortic surgery, and neurosurgical procedures upon the spinal cord. However, it has also become apparent that SSEP have limitations concerning their ability to monitor the entire spinal cord. SSEP travel exclusively in ascending sensory pathways (dorsal columns and posterolateral tracts). Accordingly, selective injury to the more anteriorly located motor tracts and motor neuronal systems in the central gray matter and anterior horn may go undetected. A number of case reports have described false negative results with SSEP monitoring, i.e., postoperative paraplegia despite unaltered intraoperative SSEP [4,27,38]. A recent survey by the Scoliosis Research Society among physicians performing intraoperative SSEP monitoring during spinal surgery revealed that five out of 27 major neurological complications (17%) that occurred with monitoring in place were not diagnosed by changes in SSEP [8]. Even if technical errors or lack of experience are taken into account that may have hampered the acquisition of reliable SSEP waveforms in some of these cases, this figure suggests that injury to the spinal cord is sometimes limited to the motor pathways. Given the differences in blood supply to the anterior and posterior spinal cord, there are several clinical situations where selective ischemia of the anterior part of the cord may ensue. This is particularly true for the thoracic spinal cord, where in some patients the anatomical variation of the anterior spinal artery may be such that interruption of a single intercostal or lumbar feeder vessel will result in spinal cord ischemia.